I work in the advertising industry and can say with confidence that for most companies the value of this sort of data is far outweighed by the negative backlash. Look for example at the way retailers initially flirted with in-store tracking and now are abandoning that technology. That fact coupled with the likely legislative reaction leads me to think that this practice won't last long.

What concerns me most is the prospect of governments doing this sort of snooping. It will probably begin with a justification that the data are valuable for managing traffic patterns and for urban planning (or other seemingly harmless purposes). But then ultimately it will be used by the government for other purposes.

It might actually be a good thing if in the short term a few companies abuse the technology to the point that the public wakes up to the amount of information broadcast by their wireless devices. Perhaps it will motivate the industry to add more security as a default setting (iCloud VPN anyone?).